Applied Pharmacology 4, Half Life of Drugs
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- čas přidán 8. 07. 2024
- Understanding half life is important to inform how and when we administer any medication. These are not casual viewing, but are for serious students who want to understand the essential underpinning concepts of the care we give our patients.
Notes
Half life t 1/2
Pharmacokinetics and Pharmacodynamics
Bioavailability - the proportion of an administered drug that reaches the systemic circulation in unchanged form
Half life is the time from peak plasma concentration until half is eliminated
Clearance
T ½ First order kinetics
First order elimination kinetics : "Elimination of a constant fraction per time unit of the drug quantity present. The elimination is proportional to the drug concentration."
Zero-order elimination kinetics : The plasma concentration - time profile during the elimination phase is linear (Fig. 1). For example 20 mg are eliminated every hour, independently of the drug concentration in the body. Order 0 elimination is rather less common, mostly occurring when the elimination system is saturated. Eg.
Alcohol
Phenytoin
Warfarin (Coumadin)
Heparin
Paracetamol (acetaminophen)
Over dose of aspirin (normally t ½ = 3-4 hours)
Steady state reached after 5 - 6 half lives (3.3 half lives for 90% concentrations), therefore possible loading dose.
Drug eliminated after 5 half lives (95% eliminated after 4.5 half lives)
Half life t ½ of common drugs
Noradrenaline 2 minutes
Salbutamol 1.6 hours
Morphine 2 - 3 hours
Methadone 24 hours
Gentamycin 2 - 3 hours
Penicillin 6 - 8 hours
Diazepam 24 hours
Fluoxetine 6 days
Warfarin 4 days
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Commenting from university of Zambia
Sir you are really a great great teacher. Thank you so much
I have a complex question. I know a bit already about pharmacology. What I always wonder is... does a drug remain in your system until say 30 half lifes have gone by? Because eventually the drug would gets down to atomic ranges? Or is the half life theory really just an arbitrary measurement. The drug is totally cleared after 6 or 7 half lifes?
ive had an injection and the half life is 29 days, does that mean i have to wait 5 months for it to be cleared from my system? thankyou
Thank you 🙏
great explanation !
thank you.
I finally got it! thanki
Hi I was wondering if you could help with a theoretical question please?
Suppose I am trying to find a painless opioid withdraw practice, what my thinking is, is exchanging a methadone patient from the long acting opioid drug like methadone and swapping it for a 2.5 hour short acting half-life opioid, then giving a long acting non-opioid painkiller to the patient, a benzodiazepine for example. would the 2.5 half-life opioid be able to get out of the system while a long acting benzodiazepine is acting on the brain, masking the pain? Hope this makes sense, thanks..
I am thinking, 48 hours after last 2.5 hour half-life opioid is taken, regardless of what other drugs have been taken, that short acting opioid should be out of the system, no?
Excellent!
what kind of pens are you using?
Thank you so much :-)
I don’t know if you will see this but I was wondering what is the half life of 5ML Diazepam? I took it about 4 and half days ago.
I know it's out of context but please help ease my mind.
Why do we care about the half life, and not the total life of the drug in the body? Is there a meaningful reason behind it?
Knowing the half life allows for a valid therapeutic range to be established. Watch again. When he shows the part using the green marker it's the therapeutic range part.
You are great Sir .
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What kind of pen are you using? Looks like a fountain pen. :)
Praises up to the Gods...Thanks!
Long question and I hope I make sense. Really hoping someone can help answer.
Question: Testosterone Cypionate has a half life of roughly 8 days, a patient takes 100 mg once every 2 weeks, in this case let's say on a Saturday. By the two week mark there is roughly only 25% of the original dose in their plasma meaning roughly 25mg circulating throughout there system. Is it fair to say that with another dose of 100mg testosterone Cypionate that there levels will technically start at 125mg in their plasma then at 8 days it would be 62mg and then at the 2 week mark be 31mg? Then an additional 100mg would be administered bringing the total to 131mg and the process would continue and continue building slowly over time?
Thanks to anyone who can shed light!
Please I need help with this question
What is the percentage of drug left after 4.5 hours if it's half life is 1.5 hours
Thanks a lot bro
Sir can u upload lecture dosing interval
Thank you so much for this video - I think I had sort of gleaned a lot of the information over the years just by reading online. I am prescribed diazepam and so am especially interested in the half life of that. Now you say that diazepam has an elimination half life of around 24hours, however, everywhere I search for the half life of diazepam it tells me 100 hours....is this because it drops below the therapeutic index within 24h but doesn't clear your system completely?
I am trying to work out best practice in order to not build up a dependence and so i am taking it every few days - does this mean that even though I am not feeling the effects of the diazepam after 24h, it is still circulating my system at a range I don't notice? In other words, does the long elimination half life mean that even though I am taking them maybe twice a week, I will still be physically addicted? I have come a long way and have tried many many medications for my multiple mental health issues (BPD((EUPD)Anxiety and depression) and I am currently prescribed 300mg of pregabalin - for anxiety (once in the morning, one in the evening - although I generally skip the evening dose tbh) - I don't really know too much about the gabagesics like gabapentin and pregabalin. I was told by the psychiatrist who took me off mirtazepine and quetiapine and put me on pregabalin that they are non-addictive but I have noticed if I go a day without taking the morning dose of 300mg pregabalin I get all sweaty and it feels like a benzo or alcohol detox - this whole benzo thing has been an issue for me ever since an irresponsible GP prescribed me temazepam and then nitrazepam when I was 16 - she even knew I had been using ketamine and cannabis (I got berated for both which is ironic given that I am now prescribed medical cannabis here nni the UK but thats a different story and this is already an essay....)
I guess, to summarise, I have two questions:
1. What is the deal with everywhere saying the half life of diazepam is 100hours but the effects only seem to last 24h?
2. Am I addicted to pregabalin? is that even possible? Was I lied to by my psychiatrist? (i'm not a paranoid guy - thats just not part of my mental health problems) but I do find myself wondering why I feel so shit if I don take the pregabalin for a few days...could it be linked to the diazepam I am taking maybe twice or three times a week? Oh and I take anywhere from 15-30mg diazepam when I do take it. I know that you are going to say that is an overdose but it just doesn't do anything at 10mg - which adds to the suspicion that my body is tolerant of diazepam physically.
I know you are not my doctor and that is why I am able to come to you, I cant tell my doctor that i'm taking more benzo's than I am meant to because she will just stop them all together and I need them every now and again - its just that 100h half life worry's the hell out of me.
I am well aware of my substances misuse disorder and have been seeking help for it for a long time but it is tricky when it comes to the fact the medications ARE actually useful to me. I try to go as many days between doses as possible and try to keep the dose low but it is hard....
Writing this all out to you, it seems like a complete mess but actually I am doing the best I have ever done, in terms of living life - I used to be an intravenous heroin user, alongside the 100mg+ of diazepam's I was taking back then. I used to buy tubs of 1000 of them and just have no self control - so I ended up tapering down slowly over months and months but I never had the courage to tell the doctor to stop prescribing me 16x 5mg tablets each month. So I was limiting myself to doses of 15mg at a time but it got to the point where I was really anxious and wasn't due a script and so bought some but I could only buy a minimum of 5x boxes! so since then (about 2 days before christmas) I have been using, as I said, 15-30mg every few days. I can't remember if I said but I am UK based also. Sorry this post is so deep, I need someone with a better understanding of drug metabolism to give me an idea of if I am likely to be PHYSICALLY dependant on both the diazepam AND pregabalin or if pregabalin is even physically addictive.
Thanks for the video and I hope you reply....
EDIT: I also forgot to mention they switched me to Espranor - a new formulation of buprenorphine (JUST buprenorphine - not like suboxone). I understand the effects of mixing buprenorphine and benzo's raise the risk of respiratory failure, however, I also know from experience, it is a totally different kettle of fish than mixing a full opioid agonist with a benzo - some thing like methadone....this is the whole reason I chose buprenorphine over methadone - that and the blocking properties it posseses at the dose I am on (8MG)
Also, I would just like to say, I am not a pharmacology student - I am actually studying art/painting, however, due toi my life circumstances, I have had to learn about this stuff, particularly as its been going on for the past 18 years of my life! I do promise I will also talk to my doctor - I do have na appointment booked but its scary to think they may just discontinue everything. Thanks again, if you respond.
great work sir
In October i'm taking my last injection of Abilify Maintena of 40 milligrams. How many months will it be in my system?
Hello doctor! thank you for your video, I am not a native speaker so could you please tell me what exactly you said on minute 4:56, did you say something like "it entire depends on the drug" ? Thank you!
Randomly stumbled upon this video.. it has me asking how therapeutic range is calculated? I guess I am asking how concentration in the blood influences how therapeutic it is
are there any drugs with this half life? just out of tangential interest.
Yes, there are many drugs with a wide variety of half lives from minutes to days
Why does It take several half lives to get into the therapeutic range?
Dear sir,I hv one doubt..
If we consider a drug, administerd enteral..drug has to absorb for a period of time. So we may not expect 1/2 amount of the drug in the blood/plasma instantly because the absortion(bioavailability)is a steady process.Then how do we disclose for 1 hr it reached t1/2, if it is for anew drug.
So this means ..t1/2 is an estimating(expecting) the bio availability for perticular time would become half when the drug under the therapeutic range
The normal convention is to start the half life time from time of peak plasma concentrations of drug. You are correct, to some extent half lives are worked out in an experimental situation, but they are still real.
Dr. John Campbell Thank you so much sir.I'm your student from India.
hi Dr. John Campbell, if you're starting to calculate the half life from the time of the peak plasma concentration, but then you're adding more, then how do you calculate it?
thank you. a good doctor is hard to find
What is the half life of lithium drug?
Excellent
A doctor from jordan
Plz can you explain..... this question.... answer
Half life (t ½) doesn’t depend on:
a. Biotransformation b.Time of drug absorption
c. Concentration of a drug in plasma
d. Rate of drug elimination
Is this an MCQ? I would go for D
Dr. John Campbell .....yes this is an MCQ..... can u plz explain... why it's D ..... I am so confused
@@pujajadhav7171 have you watched my 'applied pharmacology' series? this is covered in detail
I think drugs that have long half life should be given once daily like penicillin as you stated 8hrs is its half life and you said 4 times daily.clarify that doctor.God bless you
It may be so that the drug is continuously kept within the therapeutic window, until the course of treatment is finished.
Dealer science.
F**K.... get on with it..... any slower and we'd be in reverse